1. Technical Field
The present invention relates to cold feed rubber extruders.
2. Description of Related Art
Cold feed rubber extruders, generally comprising a feed-opening, a compression-section, a plasticizing section and an exit section leading to an extrusion head, have been replacing warm feed rubber extruders throughout the Industry, though for certain compounds and applications the latter still persist.
With advances in design, mainly in the plasticizing section, the range has been extended to include more rubber compounds which had been found difficult to extrude on account of high viscosity (Mooney values), nerve (natural rubber compounds), hardness, high fillercontent or special sensitivity to heat build-up.
Such a plasticizing section has been the Transfermix (U.S. Pat. No. 2,744,287 Parshall & Geyer 1956, U.S. Pat. No. Re. 26,147 of 1967 or UK Pat. No. 842,692 Frenkel of 1956 or U.S. Pat. No. 4,136,969 Meyer of 1979 or U.S. Pat. No. 4,184,772, Meyer of 1979) having the generic feature that in a transfer-zone formed over a common length of screw and barrel, in the screw a helical groove varies in cross-section from full area to zero area and in the barrel the cross-section of an opposite handed helical groove varies from zero area to full area, whereby in the said transfer-zone the compound is transferred from the said screw into the said barrel, while being mixed and plasticized. The said plasticizing section may comprise only one such transferzone or more, where a second one is conversely adapted by way of changes in cross-sections of helices to effect a return-transfer from barrel to screw.
For a given cross-sectional area between the bottom of the screw-helix and that of the barrel helix, such a Transfermix geometry provides a uniform action of flow division and rearrangement, treating on one transfer each subdivision just once and none repeatedly, unlike other plasticizing systems, which operate non-uniformly to different degrees and are therefore less effective.
Mathematics can be developed to show that for the earlier (1956) generation of Transfermix having invariant numbers of helical grooves in each transfer-zone, this number is limited by the condition of maintaining continuity of transport by shear stress transfer in the rubber. Expressed simply, this means avoiding grooves which get too deep and narrow so that there can be no more forward transport in the bottoms of the grooves due to the relative rotation of the screw and barrel and the grooves are no longer self-cleaning.
This limitation is, to a large measure, overcome in the second (1979) generation of Transfermix, where the number of helical grooves varies inversely as between screw and barrel within the same transfer-zone, whereby the width/depth ratio of individual helical channels can be kept such that transport flow--and thereby the important practical feature of self-cleaning--is maintained while effecting a much more intense flow-division and rearrangement, which in equal intensity can be realized for any diameter of Transfermix. Hence the designation "Multi Cut Transfermix" and the considerable widening of its plasticizing action into the range of difficult-to-extrude compounds, as well as its solution of the scale-up problem, enabling an equal plasticizing action to be designed into any diameter of a range of extruders similar in outline dimensions, such as L/D ratio.
To quote numbers on the example of a 90 mm Transfermix: In the first generation the maximum number of helical grooves in both screw and barrel for an output typical for this size would be 4-6 in the screw and 4-6 in the barrel, making 16 to 36 subdivisions, whereas the Multi Cut Transfermix would bring about 40 to 80 subdivisions.
This concentrated action, compared to other types of plasticizing section, has for both generations led to transfer-zones of a length of 1 to 1.5 screw diameters, which has allowed "easy to extrude" compounds to be plasticized homogeneously at relatively low temperatures with throughput restricted by the die-resistance only, while for more "difficult" compounds various degrees of throttling in addition to the die-resistance bring uniformity into the plasticization, thus greatly extending the range of compounds.
A disadvantage noted in the compression-section as an area serving a Transfermix plasticising section has been the following:
According to experience with Transfermix systems gained particularly on tough natural rubber (NR) and highly loaded compounds, a plain screw-compression-section leads to very high torques on the screw by leaving practically the total work of plasticization to the Transfermix-section--so much so that in certain cases a drive installed for normal compounds was stopped. Some relief was obtained by notching the screw flights over a part of their depth and at a flight-angle higher than that of the groove, not only in the entry-zone, where this is a useful measure for gripping the feed, but additionally up to about three-quarter way into the compression zone, which had a preplasticising effect and made the drive adequate again.
This, however, reduced the pressure-build up and could not produce a pre-plastication extending over the full depth of the screw channels.
Of other known plasticizing systems, especially the type called Pin Barrel Extruder (U.S. Pat. No. 4,178,104 Menges et al of 1979 corrected to 4,199,263 of 1980) has found wide application in the last 15 years. In this, at various positions along the length of a screw, rings of radially arranged flow-interruptors, mostly pins of circular cross-section, extend from the barrel into corresponding circular cuts in the screw, reaching to the bottom of the helical grooves. In each such ring, the diameter of the pins must be sufficient to withstand the considerable forces exerted by the flow of the rubber and the number of pins is limited by the fact that they must avoid throttling the forward flow.
With rotation and corresponding forward transport of compound, these pins, which loosely fit into the cuts in the screw, exert a kneading action which, on account of the above limitations on diameter and number of pins, is non-uniform around the circumference.
This kneading action is repeated in a number of so-called pin-planes, each at least one pitch-length of the screw away from the next, and has been used for plastification in extruders up to large diameters. The number of such pin-planes ranges from about 6 for easy-to-plasticize compounds to 10 or more for more difficult compounds, where excessive temperature development has generally limited the application of such extruders at the difficult end of the range of compounds.
It can be observed that in a pin plane, some elements of the flow of material are being deformed, rather than sliced, circumferentially while others slip undeformed between the pins, whereas in a Transfermix this flow is being sliced circumferentially into elements, each one a part of a thin shell, of substantially equal area and distributed over the radial depth of the screw in an orderly fashion. The deforming action in a pin plane, having regard to the necessarily small number of pins around the circumference in order to avoid throttling the flow, can not be anything like as concentrated as in any Transfermix that way, nor can this be lengthwise of the screw on account of the intervening lengths of screw between the pin-planes.
In each pin plane, however, the rotation of the compound is reduced relative to that of the screw so that on being gripped again by the rotating screw thereafter, the pressure-build-up is increased locally and forward transport tends to be improved.
This improvement in transport-action over that by the screw on its own is very limited for these reasons:
In order to be effective as a kneading element; the fit of the pins in the cuts of the screw has to be very loose. Furthermore, the number of starts of the screw that reinduce rotation of a segment of the compound is generally not above two. As a general comparison, in an axial flow pump a large number of vanes would be mounted in the rotor for inducing pressure-build up at as many circumferential positions as possible.
In the second generation of Transfermix, the problems of plasticization have been solved by providing a multiplicity of flights in the screw and the barrel in special arrangements, where at the beginning of the plasticizing zone the screw already has many more flights than in the preceding compression-zone.
It is an object of this invention to provide, in combination with a Transfermix plasticizing section, a means for effecting at the end of the compression zone a pre-plasticizing effect which extends over the full depth of the helical grooves.
It is a further object of this invention that such a preplasticizing effect be achieved with a substantial increase of the pressure build-up immediately upstream.